1. Field of the Invention
The present invention relates to a fixing device, a fixing device control method, and an image forming apparatus, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, a method of heating control for use in such a fixing device, and an electrophotographic image forming apparatus incorporating a fixing device with a heating control capability.
2. Description of the Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and setting the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image onto the medium under heat and pressure.
One such type of fixing device includes a roller-based fuser assembly that employs a fuser roller equipped with an internal heater to heat its circumference to a given process temperature. The fuser roller is paired with a pressure roller pressed against the outer circumference of the fuser roller to form a fixing nip therebetween, at which a toner image is fixed in place with heat from the fuser roller and pressure from the pressure roller.
Another type of fixing device includes a multi-roller, belt-based fuser assembly that employs an endless, flexible fuser belt entrained around multiple rollers, one of which is a fuser roller having a circumference thereof formed of elastic material, and another of which is a heat roller having a circumference thereof subjected to heating to in turn heat the length of the fuser belt rotating therearound. The fuser belt is paired with a pressure roller pressed against the fuser roller via the fuser belt to form a fixing nip therebetween, at which a toner image is fixed in place with heat from the fuser belt and pressure from the pressure roller.
The inventor has recognized that those types of fixing device experience varying environmental and operational conditions during operation, which can cause dimensional variations in the fixing members, in particular, the fuser and pressure members forming a fixing nip therebetween, leading to variations in fixing performance with which a toner image is processed through the fixing nip.
For example, in a belt-based fixing device employing a fuser belt entrained around a motor-driven, rubber-covered fuser roller, a cumulative amount of heat and pressure applied to the recording medium conveyed at a constant speed through the fixing nip is influenced by changes in the operating temperature which cause the elastic material of the fuser roller to thermally expand and contract.
Specifically, a higher operating temperature causes thermal expansion of the fuser roller to increase the length and depth to which the fuser roller engages the pressure roller, resulting in an increased amount of heat and pressure applied to the recording medium during passage through the fixing nip. Contrarily, a lower operating temperature causes thermal contraction of the fuser roller to decrease the length and depth to which the fuser roller engages the pressure roller, resulting in a decreased amount of heat and pressure applied to the recording medium during passage through the fixing nip.
Since good melting and fusion of toner to the recording medium depends on consistent application of a sufficient amount of heat and pressure through the fixing nip, variations in the fixing nip, in particular, a reduction in heat and pressure applied to the recording medium, adversely affect performance of a fixing device. In general, an excessively low operating temperature causes significant defects in a resulting image due to insufficient heating through the fixing nip, which tends to occur where printing is performed under non-steady state conditions, e.g., immediately upon power-on, using a fixed operating temperature originally designed for steady state conditions. On the other hand, fixing performance improves with increasing operating temperature causing thermal expansion of the fuser roller, insofar as the operating temperature is maintained within a normal, appropriate range.
Although thermally-induced variations in the fixing nip are also experienced by a roller-based fixing device as well, the resulting effects on fixing performance are more pronounced in the belt-based design than in the roller-based design, since the former typically employs a thick rubber-covered fuser roller with no dedicated heater provided therein (particularly in applications for high-speed color printers), which is highly prone to dimensional variations due to changes in the operating temperature.
To date, various methods have been proposed to provide a fixing process controllable against changes in environmental and operational conditions.
For example, the image forming apparatus may be given a feedback controller to control heating in a fixing device based on a temperature detected during operation. The image forming apparatus includes a thermometer disposed downstream from a fixing nip to detect temperature of a recording medium passing through the fixing nip. The temperature of the recording medium detected by the thermometer, which is assumed to indicate an amount of heat present in a fuser roller, is fed back to the controller, which accordingly adjusts a fixing temperature with which another, succeeding recording medium is processed through the fixing nip.
Alternatively, an image forming apparatus may employ a feedback controller to control pressure in a fixing nip based on a temperature and humidity detected during operation. The image forming apparatus includes an environment sensor or hygro-thermometer to detect temperature and humidity adjacent to the fixing nip, as well as an additional, auxiliary thermometer to detect temperature at a circumference of a pressure roller. The temperature and humidity detected by the sensors are fed back to the controller, which accordingly control a biasing mechanism that presses the pressure roller against a fuser member to establish the fixing nip therebetween.